This invention relates to a hydrogen conducting ceramic membrane having improved stability in mixed gases of the type encountered in fuel cell operation. The invention further relates to ceramic membrane materials having improved sinterability and density.
A variety of metallic, ceramic and polymer membranes have been used for H2 separation from gas streams. The most common metallic membrane materials are palladium (Pd) and palladium alloys (see, for example, U.S. Pat. Nos. 6,066,592 and 5,652,020). However, these materials are unsuitable for H2 separation from raw syngas due to the fact that they are poisoned by hydrocarbons at concentrations as low as 0.5 parts per million (ppm). Further, oxygen concentrations higher than 50 ppm can lead to the catalytic oxidation of hydrogen to water in the presence of Pd, resulting in localized hot spots and premature failure of these membranes. A number of organic membranes (e.g. Nafion) have also been identified as protonic conductors, but these are for lower temperature applications (less than 150° C.) and even at those temperatures are severely degraded by CO gas.
In recent years ceramic membranes have been investigated for use in purifying hydrogen gas from gas streams such as syngas. For example, U.S. Pat. No. 5,387,330 by Taniguchi et al. has shown that perovskite type oxides such as BaCe1−xMxO3, where M is a metal dopant, have high proton conductivity at elevated temperatures. Although these compounds are mixed ionic/electronic conductors, their electronic conductivity is so low relative to the ionic conductivity that they have not been useful in pressure driven type membrane devices.
More recently, Wachsman et al., in U.S. Pat. Nos. 6,235,417 and 6,296,687, claim that with appropriate substitutions in the perovskite BaCe1−xMxO3 on the “M” site the electronic conductivity could be enhanced appreciably. By substitution of the proper multivalent transition metal and lanthanide cations the electronic conductivity improved to the point where hydrogen fluxes through the mixed conducting membrane were comparable to the O2 flux achieved through the analogous mixed conducting oxygen ion membranes based on La1−ySryCo1−xMxO3. Wachsman et al., however, has not resulted in an economically feasible process for purifying hydrogen from gas streams. Other problems remain that have prevented commercialization, namely, the hydrogen flux rates are still not sufficient to produce a commercially viable device and the membranes have not been shown to have adequate thermochemical stability in the syngas environment.
While alternate dopants have resulted in an increase in the electronic conductivity, the increase in electronic conductivity is insufficient for the membrane to function effectively as a pressure-driven hydrogen separation membrane. Alternatively, if a two-phase composite can be fabricated wherein an electronically conducting phase and a protonic conducting phase form interpenetrating networks within a dense ceramic, it may be possible to independently control the fluxes of protons and electrons. Argonne National Laboratory (ANL) is developing dense ceramic/metal composites to fabricate mixed conducting membranes for hydrogen separation. [J. Guan et al., “Development of Mixed-Conducting Ceramic Membranes for Hydrogen Separation,” Ceramic Transactions 92 1-12 (1998) and R. V. Siriwardane et al. “Characterization of Ceramic Hydrogen Separation Membranes with Varying Nickel Concentrations,” Applied Surface Science 167 [1] 34-50 (2000)]. The ANL group has reported hydrogen fluxes as high as 15 cc/cm2/min when the metallic phase is also a hydrogen conductor, i.e. palladium. However, Pd-based cermets are not good options for H2 separation from syngas as Pd is significantly poisoned by hydrocarbons at high temperature. With a non-hydrogen conducting protonic phase, the best fluxes published by the ANL group is below 1 cc/cm2/min. Further, the selection of an appropriate metallic second phase with thermochemical and thermomechanical stability is also a concern. Most metals are embrittled by H2 or corroded by the trace components in syngas at elevated temperatures. Relatively inert metals such as platinum and gold have thermal compatibility issues with the protonic conducting phase due to thermal expansion mismatch. In addition, metals are generally soft at the elevated temperatures, which make the puncturing of very thin membranes of the order of 20 μm very likely. Therefore, while the use of cermets for pressure-driven H2 separation remains an interesting possibility, further technological advances are required to make it a viable technology.